1. Field of the Invention
The present invention relates to open-head power tongs used in drilling operations, and more particularly, is directed to an improved means of actuating and deactuating the operation of the power tong drive means in response to the opened and closed positions of an access door.
2. Description of the Prior Art
As well known in the drilling industry, power tongs are employed in making-up and breaking-out operations of casings, tubings, rods, pipes and the like. More particularly, power tongs are used to grip and rotate lengths of drill pipe or the like to connect or join several lengths of pipe together to thereby form a drill string in a make-up operation, and in the alternative, to grip and rotate a length of drill pipe to disconnect it from the drill string in a break-out operation.
One type of power tong commonly used today is the open-head tong, such as the one shown and described in U.S. Pat. No. 4,060,014. The open-head tong has a bifrucated frame defining a central opening and a side opening communicating with the central opening for the passing therethrough of a drill pipe or the like. Due to the extreme costs of drilling, open-head tongs have become very popular, in that, they can easily and readily be moved into and out of an operative position when they are needed in the making up and breaking out of drill strings.
In operation, the open-head power tong exerts large rotational torques on the drill pipes, usually the larger the tong, the larger the torque output. Due to these large torque outputs and the resulting forces generated therefrom, the open-head tongs have been provided with an access door that bridges the gap between the bifrucated ends of the tong. The primary purpose of such an access door is to strengthen the tong structure so as to prevent, during the operation of the tong, the bifrucated ends from separating or springing apart, which not only results in damage to the tong, but could also inflict injury to the operating personnel. The access door, in addition to providing structural rigidity to the tong, also provides the operator with safety in bodily protecting him from the rotating pipe gripping and engaging jaws.
Such access doors perform very satisfactorily in providing structural rigidity to the tong and do provide protection to the operators from the rotating components of the tong when the door is properly latched in position during the make-up and break-out operations; however, in an effort to save time, operators have been known to operate the tong with the access door open, and in some instances, the operators have even removed the access door from the tong. Such operator's carelessness not only causes costly structural damage to the tong, but also results in personal injury to the operator.
In U.S. Pat. No. 2,705,614 there is shown an open-head power tong having an automatic hydraulically powered access door, operably interconnected to the hydraulic cylinders that actuate the jaw gripping mechanism, which must be closed before the jaws can be actuated so as to rotate a drill pipe. Such door interlock mechanism has been specifically designed for the type of tong disclosed and is not readily adaptable to other types of power tongs, such as the one shown in the above-mentioned U.S. Pat. No. 4,060,014. Further, the hydraulic circuitry that is involved with such a powered access door is not only complicated, having expensive components, but is also, costly to maintain and repair. Still further, such door interlock mechanism does not provide adequate safety to an operator, in that, although the operator is protected from the pipe gripping and engaging mechanism when the door is closed, he is also subjected to the risk of having the power operated door being automatically swung into him as it is being closed, thus, creating a potentially dangerous and unsafe condition under which the operator must work.